That's a common error. It happens because you are trying to load the .js script before the textures (a .zip file). Try to do the opposite and it will work. Meaning, try to load the textures and then load the js script.
Because the mod uses the cow entity to make the companions. Maybe doesn't it make much sense to you but it's usually how most new mobs in Minecraft made (I think!).
// Edited by Editor